Freeze-dried ammonia borane-polyethylene oxide composites: Phase behaviour and hydrogen release

A.R Ploszajski,M Billing,A.S Nathanson,M Vickers,S.M Bennington

doi:10.1016/j.ijhydene.2018.01.128

A.R Ploszajski, M Billing + Show 3 more

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https://doi.org/10.1016/j.ijhydene.2018.01.128

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Abstract

A solid-state hydrogen storage material comprising ammonia borane (AB) and polyethylene oxide (PEO) has been produced by freeze-drying from aqueous solutions from 0% to 100% AB by mass. The phase mixing behaviour of AB and PEO has been investigated using X-ray diffraction which shows that a new ‘intermediate’ crystalline phase exists, different from both AB and PEO, as observed in our previous work (Nathanson et al., 2015). It is suggested that hydrogen bonding interactions between the ethereal oxygen atom (–O–) in the PEO backbone and the protic hydrogen atoms attached to the nitrogen atom (N–H) of AB molecules promote the formation of a reaction intermediate, leading to lowered hydrogen release temperatures in the composites, compared to neat AB. PEO also acts to significantly reduce the foaming of AB during hydrogen release. A temperature-composition phase diagram has been produced for the AB-PEO system to show the relationship between phase mixing and hydrogen release.

Highlights

Ammonia borane (AB, NH3BH3) is a highly promising candidate material to meet the US Department of Energy's targets for portable hydrogen storage systems owing to its excellent gravimetric hydrogen storage capacity of 19.6 wt% H2, low dehydrogenation temperatures, and encouraging progress in recycling the material [2].the commercialisation of ammonia borane (AB) has been hampered by its low melting point (114 C [3]) which, when hydrogen is produced at proximate temperatures, results in a waxy foam that is difficult to contain in a practical device
Some of the patterns contain several small peaks from an unidentified phase or phases and are most noticeable in the XRD from AB40. These peaks are present in the pattern for AB100, so it is likely that they originate from impurities from the as-received AB, or hydrolysis products formed during synthesis
Borazine's boiling point is 53 C, it is thought that the molten polyethylene oxide (PEO) could act as a solvent, coordinating to borazine molecules, and allowing for their detection in 11B{1H} SS MAS-NMR. This evidence suggests that the AB within the intermediate phase undergoes the low-temperature dehydrogenation reaction due to the hydrogen bonds formed between the ethereal oxygen atom in the PEO backbone and the protic hydrogen atoms attached to the nitrogen atom (NeH) of AB molecules

Summary

Introduction

It is thought that the AB in the intermediate phase is effectively dissolved in a viscous PEO solution before and during its low temperature hydrogen release reaction. In the in-situ 11B{1H} SS MAS-NMR data for AB30 (Fig. 8b), in which AB is thought to only exist in the intermediate phase, the formation of DADB is observed, but its existence is shortlived compared with the DADB signals for AB70 and AB100 (Fig. 8c and a), likely because the highly polar DADB is unstable in the PEO solution.

Results

Conclusion